A rotating turbine blade, also known as a turbine bucket or turbine rotor blade, converts energy from a flowing fluid such as hot combustion gas or steam into mechanical energy by causing a shaft of a turbomachine to rotate. As the turbomachine transitions through various operating modes, the turbine blades are subjected to both mechanical and thermal stresses.
Mechanical stresses, such as fatigue, may be caused by fluctuating forces in combination with steady state forces. More specifically, the turbine blades may experience fluctuating forces when they rotate through non-uniform fluid flow downstream from stationary vanes, also known as nozzles, positioned between adjacent rows of turbine blades. A basic design consideration for turbomachines is to avoid or to minimize resonance with natural frequencies of the turbine blades and the dynamic stresses produced by forced response and/or aero-elastic instability.
For example, each turbine blade on a rotating turbine disc experiences a dynamic force when rotated through the non-uniform flow from stationary vanes. As the turbine blades rotate through areas of non-uniform flow, they may exhibit a dynamic response, such as, for example, stress, displacements, etc. Additionally, a turbine bladed disc may be induced into a state of vibration wherein the energy build up is a maximum. This is exemplified by areas of the blade or disc where the stress or displacement is at a maximum level, and the resistance to the exciting force of the blade or disc is at a minimum. Such a condition is known as a state of resonance.
When analysis or empirical testing indicates that a turbine blade and/or rotor disk may encounter a resonance condition during operation of the turbomachine, steps may be taken to facilitate minimizing the probability of encountering resonance. For example, shroud sets may be formed along the span of each of the turbine blades. Each shroud set generally includes a pair of circumferentially extending shrouds, one shroud projecting from a suction side surface of a turbine blade and one shroud projecting from a pressure side surface of the same turbine blade. Because the shrouds are located intermediate to a blade root portion and a blade tip portion of each turbine blade, they are often referred to as mid-span shrouds. However, mid-span shrouds can be located anywhere along the turbine blade span, not just at the physical mid-point of the span.
Mid-span shrouds are generally effective for avoiding or minimizing resonance with natural frequencies of the turbine blades and/or the dynamic stresses produced by fluctuating forces or “flutter”. However, mid-span shrouds are typically cast as part of the turbine blade and may require additional machining or other finishing processes to produce a finished turbine blade. This may only be cost-effective during a design phase of the turbine blade. In addition, a cast in mid-span shroud may not be retrofitted to pre-existing turbine blade designs.
Another method for providing mid-span shrouds to the turbine blade includes press fitting a support member through a bore hole defined in the turbine blade and connecting each shroud to the support member. However, this method may result in undesirable stresses on the turbine blade and/or may result in the support member becoming loose within the bore hole due to differences in thermal expansion between the turbine blade and the press-fit support member during operation of the turbomachine. Therefore, a non-cast or non-integral mid-span shroud assembly which connects to a new or pre-existing turbine blade to alter frequency and mode shape in order to mitigate flutter and/or modify bucket vibratory characteristics would be useful.